1. Field of Invention
The present invention relates to a material of negative electrode in battery and its preparation method, and particularly to a composite carbon material of negative electrode in lithium ion battery.
2. Description of Related Arts
From the 60's of last century, as the oil crisis and with the increasing of the population, resources shortage becomes a serious problem. In recent time, environment protect is also been concerned by people, so that green power has developed rapidly. Lithium ion (Li-ion) battery is a new generation of secondary battery after the Nickel metal hydride (NiMH) developed in 90's of last century. With the advantage of high voltage, large capacity, long cycle life, small self discharge current, no memory effect, etc., Li-ion battery is the first choice for most high-end consumer electronics and is also widely used in cutting edge technique field such as aerospace and military area. Nowadays, Li-ion battery is one of the most popular fields of research and development.
In current technique, the negative electrode of commercial Li-ion battery is mad of carbon from mostly petroleum coke and natural graphite. Particularly, natural graphite has the advantage of high capacity, good performance, extensive resource, low cost, and is widely used. But the natural graphite has high degree of graphitization. The crystal structure and the physical chemical properties of the peripheral and the bottom of the graphite microcrystalline vary largely. The reaction with the electrolyte is unevenly while the decomposition of the electrolyte mainly occurs at the peripheral of the graphite microcrystalline. As a result, the density of the passive film is poor. During the charging process, it is easy to embed the lithium ion solvate, and causes the swelling and collapse of the graphite layer, increases the irreversible capacity. Also, through the physical and chemical treatment, the attachment of the natural graphite to the electrode plate is relatively weak. During the charging-discharging cycle process, it is easy to fall out and affect the cycle life, especially with large charging-discharging current. Moreover, as the negative electrode of Li-ion battery, the natural graphite needs to be crushed in order to increase the surface area to store more lithium. To enable the embedding-moving out process, the graphite needs to be crushed less than 100 μm. As a lubricating material, graphite is pones to have sliding between layers, resulting in the smashing of crystal structure, and finally affect the charge-discharge capacity and high current change and discharge performance.
In order to improve the electrochemical properties of the graphite material, different techniques are used to change the characters of natural graphite and to modify the surface structure. Japan patent No. 2000-261046 published a method of thermal oxidation processing graphite powder, to change the state of the surface. Although the reaction of electrolyte and anode is improved, the discharging capacity is still low. U.S. Pat. No. 6,403,259 published a method of grinding natural or artificial graphite to coat a layer of carbon material. The method improves the properties of high temperature self discharging, and low temperature properties, but other aspects of the performance is unsatisfied.